1. Field of the Invention
This invention relates to stablized water slurries of carbonaceous materials and more particularly to coal-water and lignite-water mixtures.
2. Description of the Prior Art
The United States and many other nations of the world have large energy reserves in the form of coal and other carbonaceous solid materials. Although the cost of coal and other solid fuels is generally relatively low at the mine site, energy consuming centers are often distantly removed from the energy sources. Accordingly, transportation costs typically form a significant fraction of the cost of using coal and other carbonaceous fuels, such as lignite.
In order to reduce transportation costs, pipelines capable of pumping coal-water slurries have been proposed. Two commercial-scale coal slurry pipelines have been constructed in the United States. One of these, the 273-mile Black Mesa Pipeline in Arizona and Nevada, is now transporting 4 million tons per year of coal from a mine in Nevada to a power plant in New Mexico. A number of other slurry pipelines, including the proposed 1670 mile ETSI pipeline to be built to carry 37,500 tons per year of coal from Gillette, Wyo. to Pine Bluff, Ark., are in various stages of design or negotiation.
The combustion of coal- or lignite-water mixtures as substitute fuel for increasingly expensive petroleum is also the subject of considerable research and development activity. Experimentation has progressed from the laboratory to tests in industrial-scale boilers, direct-fired heaters and electric power generation facilities.
For both slurry transport and CWM combustion, raw coal is crushed and blended with 30 to 50% additional water to produce a stable, pumpable mixture. The maximum coal particle size at the Black Mesa operation is approximately 2 mm, and nearly 20% is purposely crushed to less than 44 microns. This fine size fraction is necessary to produce, with the water present, a "vehicle" to minimize sedimentation of the coarser size fractions. In a typical CWM, the maximum particle diameter is approximately 150 microns, and approximately 80% of the particles have diameters less than about 75 microns. In both applications (transport and combustion), every effort is made to minimize the amount of water used, since in the case of direct combustion of CWM, an energy penalty is paid for the presence of water, and too high a water content causes problems in maintaining a stable flame. In the case of slurry pipelining, water may be scarce at the slurry preparation site, separation of water from the coal at the pipeline terminus is expensive, disposal of slurry water may pose environmental problems, and transportation of this "inert" material adds to pumping costs.
However, aqueous slurries of finely ground coal containing over 65 wt. % solids are currently difficult to pump. It is well know that the apparent viscosity of a concentrated CWM at a given shear rate is not linearly related to the solids fraction. As the solids content approaches that corresponding to dense packing, further small increases in solids content greatly increase the viscosity and thus the pumping costs and reducing porability. In the CWM, this phenomena is observed at a solids content of about 65% by weight. Although these problems can be avoided by decreasing the weight percent of solids, a high water content is undesirable, as is discussed above.
Interest in both the transportation of coal by slurry pipeline and in the direct combustion of coalwater mixtures (CWM) has reached a high level in recent years. The development of similar technology for lignite-water slurries and suspensions is lagging far behind that of coal. This lag is primarily due to the high water-to-solids ratio required of lignite-water mixtures (LWM) that are to be pumped at reasonable energy cost or that are to be used as a substitute for oil in process heating.
U.S. Pat. No. 4,282,006 discloses pipeline pumped high-solids content, coal-water slurries. These slurries may be generally characterized as having a Brookfield viscosity of about 2,000 cps or less at 60 rpm at about 75 wt. %, dry basis, coal content. The primary factor contributing to high solids content and fluidity is said to be the particle size distribution of the slurries, although dispersing agents and/or electrolytes are also used in the preparation of these slurries.
U.S. Pat. No. 4,417,902 describes a process for the production of coal-water slurries by the incorporation therein of a prescribed group of additives. These additives reduce viscosities at high rates of applied shear. The additives are selected from .beta.-substituted carboxylic acids, dialkanol alkyl amines, and diamines of lower molecular weight dissolved or dispersed in the slurry.
U.S. Pat. No. 4,436,528 also discloses the use of dispersants to decrease viscosity but additionally includes a water-soluble polymer selected from specified alginates and gums.
U.S. Pat. No. 4,441,888 also describes a coal slurry containing a dispersing agent and a water-soluble polymer, but uses a olefin/maleic acid copolymer as the water-soluble polymer.
U.S. Pat. No. 4,457,762 describes a coal-water slurry containing a condensation product formed from arylphenol sulfonic acids and related compounds.
The cited patents are representative of a larger group of patents which describes the production of stabilized water slurries of carbonaceous materials. Two approaches have generally been taken to produce a high solids content while retaining low viscosity. One involves the careful optimization of the coal particle size distribution. For a given solids content, the particle size distribution that gives the lowest mixture viscosity appears to be one that has a broad range of particle sizes. The other method used to minimize viscosity involves the use of chemical additives for dispersion and stabilization of the suspension. The cost of these additives, typically a hydrocarbon-based carboxylic acid or xanthan gum, are in the range of $5 to $15 per ton of coal-water mixture (CWM). Because of the high added expense of dispersants (relative to total fuel price) and because of the difficulty in accurately controlling particle size distribution at the pipeline head, additional means for controlling the viscosity of coal-water and lignite-water slurries remain needed.